Phosphate-treated galvanized steel sheet excellent in corrosion resistance and paintability

ABSTRACT

The present invention provides phosphate-treated galvanized steel sheet excellent in corrosion resistance and paintability, including: 
     a phosphate-treated galvanized steel sheet excellent in corrosion resistance and paintability, comprising a phosphate film in an amount of at least 0.5 g/m 2 , containing at least 2 wt. % of Mg, and at least 0.5 wt. % in total of Ni and/or Mn, with at least 4 wt. % in total of Mg and Ni and/or Mn, formed on a zinc or zinc alloy plated steel sheet; and a phosphate-treated galvanized steel sheet excellent in corrosion resistance and paintability, comprising a phosphate film in an amount of at least 0.3 g/m 2 , preferably at least 1 g/m 2 , containing at least 2 wt. % of Mg, and at least 0.5 wt. % in total of Ni and/or Mn, with at least 5 wt. % in total of Mg and Ni and/or Mn, formed on a zinc or zinc alloy plated steel sheet.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a phosphate-treated galvanized steelsheet excellent in corrosion resistance and paintability for use in suchapplications as automobiles, home electric appliances and buildingmaterials.

2. Description of the Related Art

It has been the most common conventional practice to subject galvanizedsteel sheets for use in applications such as automobiles, home electricappliances and building materials to a phosphate treatment, a chromatetreatment, and further, an organic coating treatment with a view toimproving added values such as corrosion resistance and paintability ofthe galvanized steel sheets. However, the recent tendencies have beenthat the chromate-treated steel sheet, in particular, which may containhexavalent chromium is avoided from environment problems, and there hasbeen an increasing demand for phosphate treatment of the galvanizedsteel sheets.

With the conventional phosphate treatment of a galvanized steel sheet,however, sufficient corrosion resistance or paintability is not alwaysachieved, and therefore various methods for the improvements have beenproposed. For example, Japanese Patent Publication No. 60-34912discloses a method in which after a phosphate film is formed, the filmis treated with an inhibitor. Japanese Laid-Open Patent ApplicationsNos. 60-50175 and 8-13154 disclose methods of achieving coexistence ofNi, Mn and the like in a phosphate film.

The aforementioned methods, while giving certain effects, are notsufficient to meet with the recent severer requirements for corrosionresistance, and provides almost no improving effect of bare corrosionresistance in particular.

Japanese Laid-Open Patent Applications Nos. 1-312081 and 3-107469disclose zinc phosphate films containing Mg. In these cases also, thecorrosion resistance improving effect is not sufficient, with aninsufficient paintability. Further, Japanese Laid-Open PatentApplication No. 9-49086 discloses a method of forming a zinc phosphatefilm containing Ni and Mg, still suffering from the problem ofinsufficient corrosion resistance.

DESCRIPTION OF THE INVENTION

The present invention has an object to solve these problems and providea phosphate-treated galvanized steel sheet excellent in corrosionresistance and paintability.

For phosphate-treatment of a galvanized steel sheet, the presentinventors have made various trials to cause Mg ion and Ni ion to becoexistent in large quantities in a treatment bath and to form ahigh-phosphate film having high contents of Mg and Ni which could not beachieved by the conventional art. As a result, it has been found thatcorrosion resistance and paintability can be improved by increasing thecontents of both Mg and Ni in the phosphate film. It has been furtherfound that a phosphate film having still higher Mg and Ni contents isachieved by coating an aqueous phosphate solution containing Mg and/orNi, after the formation of the phosphate film containing Mg and Ni, anddrying the resultant coated sheet without rinsing with water. Thesefindings have led to a conclusion that, by maintaining the Mg and Nicontents in the resultant phosphate film within a specific rangerespectively, it is possible to obtain very good corrosion resistanceand paintability so far unknown. It has been still further found thatthe same effect can be achieved by Mn in place of Ni.

The present invention has been completed on the basis of theaforementioned findings and the gist of the present invention is toprovide a phosphate-treated galvanized steel sheet excellent incorrosion resistance and paintability, comprising a phosphate film in anamount of at least 0.5 g/m², containing at least 2 wt. % of Mg and atleast 0.5 wt. % of Ni and/or Mn, with Mg and Ni and/or Mn being intotal, formed on the surface of a steel sheet coated with zinc or a zincalloy; and further provide a phosphate-treated galvanized steel sheetexcellent in corrosion resistance and paintability, comprising aphosphate film in an amount of at least 0.3 g/m², preferably at least 1g/m², containing at least 2 wt. % of Mg, at least 0.5 wt. % of Ni and/orMn, with Mg and Ni and/or Mn being in an amount of at least 5 wt. % intotal, formed on the surface of a steel sheet plated with zinc or a zincalloy.

DESCRIPTION OF PREFERRED EMBODIMENTS

There is no particular limitation regarding the galvanized steel sheetused in the invention, and the invention is applicable to both pure zinccoated and zinc alloy coated steel sheet. Any zinc and zinc alloycoating method including electro-galvanizing, hot-dip galvanizing andvapor deposition may be applicable.

According to the present invention, the phosphate film formed on thegalvanized steel sheet normally contains Zn dissolved from the zinccoating or coming from the phosphate treatment bath, but it is essentialthat the film contains Mg and Ni and/or Mn. The required ratios of thesemetal contents to the weight of the phosphate film as a whole are atleast 2 wt. % for Mg, at least 0.5 wt. % for Ni and/or Mn, and at least4 wt. % for the total of Mg and Ni and/or Mn. Any of these metalcontents below the lower limits results in remarkable deterioration ofcorrosion resistance and paintability. Mg, Ni and/or Mn shouldpreferably be at least 5 wt. % in total.

There is no particular limitation for the upper limit of the contents ofthe above metals. However, the content of Mg and Ni singly or incombination is limited up to about 10 wt. %, and Mg and Mn and/or Ni intotal is limited up to about 15 wt. %. It is technically difficult tomaintain their contents over these upper limits.

The phosphate film containing at least 4 wt. % of Mg and Ni and/or Mn intotal must be present in a weight of at least 0.5 g/m², below which nosatisfactory corrosion resistance can be obtained. Also the phosphatefilm containing at least 5 wt. % of Mg and Ni and/or Mn in total musthave a weight of at least 0.3 g/m², below which no sufficient corrosionresistance can be obtained, and which should more preferably be at least1 g/m². Although there is no particular limitation on the upper limit,it should preferably be in general up to about 2.5 g/m2 when takingweldability into consideration.

Since Ni and Mn produce the same effects, the following description willcover only the case where Ni is used without using Mn.

The phosphate film containing Mg and Ni according to the invention canbe obtained by the treatment using a phosphate bath containing Mg ionand Ni ion. Prior to such a treatment, it is desirable to perform aknown pretreatment such as a titanium colloidal treatment or a brushingtreatment. An example of the phosphate treatment bath may be illustratedby a bath prepared by adding Mg ion and Ni ion to a treatment bathcontaining Zn ion, phosphate ion, fluoride, an oxidizing agent (such asa nitrate, a nitrite or a chlorate). In this case, the concentration(weight percentage) of the metal ions relative to one Zn ion should bepreferably about 10 to 50 for Mg ion, and about 1 to 10 for Ni ion. Byusing such a treatment bath, it is possible to form the phosphate filmon the galvanized steel sheet by a spraying or dipping method, forexample. However, by using such a method, if Mg and Ni are to coexist,their weight percentage in total in the resultant film is technicallylimited up to about 5% maximum. Trying to add them in a higherpercentage is not only difficult, but also tends to cause defectiveprecipitation of the film or production of much sludge.

In order to achieve coexistence of Mg and Ni in larger amounts, it isdesirable to apply first a phosphate treatment using the bath containingMg and Ni as described above, or a usual phosphate treatment using abath not containing Mg or Ni, and then, to coat the thus treated sheetwith an aqueous phosphate solution containing Mg and/or Ni, and dryingthe thus coated sheet without water rinsing to a sheet temperature of 90to 150° C. without water rinsing, thereby forming a composite phosphatefilm. As the aqueous solution to be coated, an aqueous solution of aprimary phosphate of the metals (known also as dihydrogen phosphate saltor biphosphate salt) is preferable, and as the coating method, aroll-coating method is preferable. The coating may be applied to theboth surfaces, or only to one surface of the sheet. Particularly for anautomobile sheet, for example, it is also appropriate to coat only thesurface of the sheet which, when used in an automobile, forms the innersurface required to have a high corrosion resistance.

When the above aqueous phosphate solution is coated, the weight of thephosphate film according to the present invention is the total weight ofthe primary phosphate treatment film plus the phosphate film formed bythe aqueous phosphate solution. And the contents of Mg and Ni in thefilm are the total contents of Mg and Ni in both the primary phosphatetreatment film and the subsequently coated phosphate film formed by thephosphate solution are expressed as percents of the total weight of bothfilms and when the total contents of Mg and Ni and the total weight ofboth films are within the specified ranges according to the presentinvention, it is possible to obtain satisfactory corrosion resistanceand paintability.

Mn may be used in place of Ni, as described above, and the same effectsand advantages can be obtained also by simultaneously using Ni and Mn.

Preferred Embodiments

Examples of the present invention will be described hereinbelow.

An electrogalvanized steel sheet having a coating weight of 30 g/m² (perone side) was employed in all of the following examples.

SAMPLE PREPARATION EXAMPLES 1 to 8

Galvanized steel sheets were subjected to a pretreatment using acommercially available titanium colloidal treatment agent (PL—Zn made byNihon Perkerizing Co., Ltd.) and then to a primary phosphate treatmentby spraying a phosphate treatment bath shown in Table 1. Phosphate filmshaving a weight as solid ranging from 0.2 to 1.7 g/m² varying accordingto the respective examples were formed at a temperature varying from 60to 70° C. for a spraying time varying from 1.5 to 10 seconds. After thetreatment, the thus treated sheets were once water-rinsed, dried, andfurther coated with an aqueous solution of magnesium biphosphate(aqueous solution of 50% magnesium biphosphate made by Yoneyama ChemicalIndustry Co., Ltd. diluted to five times) by roll-coating. The coatedsheets were dried so as to achieve a final sheet temperature of 110° C.The coated film dry weights were adjusted to 0.3 to 1.5 g/m² by varyingthe number of rotation of the coater.

TABLE 1 Composition of Phosphate Treatment Bath {circle around (1)}Concen- tration Phosphate ion 10 g/l Zn ion 2 g/l Ni ion 4 g/l Mg ion 1g/l Nitrate ion 10 g/l F 0.3 g/l Total acid degree/free acid degree 15

EXAMPLE 9

A phosphate film of 0.7 g/m² after drying was formed in the same way asin the preceding examples by the using the phosphate treatment bathshown in Table 1 under conditions of a spraying time of 2 seconds and atreatment bath temperature of 60° C. After water rinsing and drying, anaqueous phosphate solution containing a solid concentration of 10%prepared herein below was further coated on the phosphate treated steelsheet with a roll coater to have a total coated film weight of 1 g/m²and the thus coated sheet was dried so as to reach a carry-over sheettemperature of

The aqueous solution was prepared by mixing a magnesium biphosphateaqueous solution (50% aqueous solution of magnesium biphospate made byYoneyama Chemical Industry Co., Ltd.) with a manganese biphosphate (madeby Yoneyama Chemical Industry Co., Ltd.; manganese phosphate dihydrogen4 hydrate) and diluting the mixed solution so as to achieve a solidweight ratio of 2:1.

EXAMPLE 10

In the phosphate treatment bath shown in Table 1, Mn ion in an amount of4 g/l was added in place of Ni ion, and a phosphate film of 1 g/m² wasformed under conditions of a spraying time of 2 seconds and a treatmentbath temperature of 65° C. After water rinsing and drying, an aqueous ofmagnesium biphosphate (50% aqueous solution of magnesium biphosphatesolution made by Yoneyama Chemical Industry Co., Ltd.; diluted to fivetimes) was coated with a roll coater, and the coated film was dried toreach a final sheet temperature of 110° C. so as to obtain 0.7 g/m² ofthe coated film in the solid form.

EXAMPLE 11

A phosphate treatment was conducted with a phosphate treatment bathshown in Table 2 by spraying. A phosphate film of 1.0 g/m² after dryingwas formed under conditions of a spraying time of 1.5 seconds and atreatment bath temperature of 60° C. After water rinsing and drying, asolution having a solid concentration of 10%, prepared by mixing anaqueous solution of manganese biphosphate (manganese phosphatedihydrogen tetrahydrate made by Yoneyama Chemical Industry Co., Ltd.)with an aqueous solution of magnesium biphosphate (50% magnesiumbiphosphate solution made by Yoneyama Chemical Industry Co., Ltd.) anddiluting the mixed solution with water, to achieve a solid weight ratioof 2:1, was further coated on the phosphate treated sheet with a rollcoater and the thus coated sheet was dried so as to reach a final sheettemperature of 110° C. to obtain a total solid weight of 1 g/m² of thecoated film.

TABLE 2 Composition of Phosphate Treatment Bath {circle around (2)}Concen- tration Phosphate ion 10 g/l Zn ion 2 g/l Ni ion 0.2 g/l Nitrateion 10 g/l F 0.2 g/l Total acid degree/free acid degree 15

EXAMPLES 12 and 13

A phosphate treatment was carried out by spraying on the steel sheet thephosphate treatment bath shown in Table 3. Phosphate films of 1.4 g/m²Example 12 and 0.6 g/m² (Example 13) respectively were prepared underconditions of a respective treating time of 4 seconds and 2 seconds anda treatment bath temperature of 70° C.

TABLE 3 Composition of Phosphate Treatment Bath {circle around (3)}Concen- tration Phosphate ion 6 g/l Zn ion 1 g/l Ni ion 4 g/l Mg ion 30g/l Nitrate ion 80 g/l F 0.3 g/l Total acid degree/free acid degree 15

EXAMPLE 14

After carrying out the same treatments as in Example 13, an aqueoussolution of magnesium biphosphate (50% aqueous solution of magnesiumbiphosphate made by Yoneyama Chemical Industry Co., Ltd.; diluted tofive times) was coated with a roll coater, and dried to reach a finalsheet temperature of 110° C. so as to obtain a solid weight of 1 g/m² ofthe coated film.

EXAMPLE 15

A phosphate treatment was applied by spraying the phosphate treatmentbath shown in Table 3. A phosphate film of 0.4 g/m² was formed underconditions of a spraying time of 2 seconds and a treatment bathtemperature of 65° C. After the treatment, the treated film waswater-rinsed and dried.

EXAMPLE 16

A phosphate treatment was applied by spraying the phosphate treatmentbath shown in Table 3. A phosphate film of 0.2 g/m² after drying wasformed under conditions of a spraying time of 1.5 seconds and atreatment bath temperature of 60° C. After water rinsing and drying, thesheet was further coated with an aqueous solution of magnesiumbiphosphate and manganese biphosphate mixed together, having their solidweight ratio of 1:1 and dried so as to reach a final sheet temperatureof 110° C. and a solid weight of 0.1 g/m²of the coated film.

COMPARATIVE EXAMPLE 1

A phosphate treatment was applied by spraying the treatment bath havingthe same composition as in Table 3 except that Ni ion concentration iszero. A phosphate film of 1.5 g/m² was formed under conditions of aspraying time of 6 seconds and a treatment bath temperature of 65° C.After the treatment, the sheet was water-rinsed and dried.

COMPARATIVE EXAMPLE 2

A phosphate film of 0.1 g/m2 was formed under conditions of a sprayingtime of 0.5 seconds and a bath temperature of 55° C. using the treatmentbath shown in Table 1. After water-rinsing and drying, an aqueoussolution of magnesium biphosphate (50% aqueous solution of magnesiumbiphosphate made by Yoneyama Chemical Industry Co., Ltd.; diluted to tentimes) was coated with a roll coater, and dried so as to obtain a finalsheet temperature of 110° C. and a coated film weight after drying of0.1 g/m^(2.)

COMPARATIVE EXAMPLE 3

A phosphate film was formed in the same manner as in Example 5 exceptthat the aqueous solution of magnesium biphosphate was not coated afterthe phosphate treatment.

COMPARATIVE EXAMPLE 4

A phosphate treatment was applied by spraying the phosphate treatmentbath shown in Table 4. A phosphate film of 1.5 g/m² after drying wasformed under conditions of a spraying time of 4 seconds and a treatmentbath temperature of 70° C. After the treatment, the film was rinsed withwater and dried.

TABLE 4 Composition of Phosphate Treatment Bath {circle around (4)}Concen- tration Phosphate ion 15 g/l Zn ion 1 g/l Ni ion 3.5 g/l Mg ion2 g/l Nitrate ion 18 g/l F 1 g/l Total acid degree/free acid degree 30

COMPARATIVE EXAMPLE 5

A phosphate treatment was applied by spraying the phosphate treatmentbath shown in Table 5. A phosphate film of 1.5 g/m² after drying wasformed under conditions of a spraying time of 4 seconds and a treatingbath temperature of 65° C. Then, the formed film was water-rinsed anddried.

TABLE 5 Composition of Phosphate Treatment Bath {circle around (5)}Concen- tration Phosphate ion 15 g/l Zn ion 1 g/l Ni ion 3.5 g/l Mg ion8 g/l Mn ion 2 g/l Nitrate ion 28 g/l F 1 g/l Total acid degree/freeacid degree 30

PERFORMANCE EVALUATING METHOD

Film weight: The phosphate film was totally stripped off by immersingthe sample in a mixed aqueous solution stripping solution of 20 g/l ofammonium bichromate and 490 g/l of 25% ammonia. The film weight wascalculated from the sample weight difference before and after stripping.

Film components (Mg, Ni, Mn): After heating the stripping solutioncontaining the film with addition of nitric acid, quantities of Mg, Niand Mn were determined with an ICP, and their weight percentagesrelative to the total film weight were calculated.

Paint Coating Adhesion (primary): Alkali degreasing (using SD280MZ madeby Nihon Paint Co.), chemical treatment (using SD2500MZL made by NihonPaint Co.) and cationic electrodeposition (using V-20 made by NihonPaint Co., having a thickness of 20 μm) were applied to the sample.After holding the sample in this state for one day, scratches reachingthe substrate were cut in checkers (100 squares) at intervals of 2 mm bymeans of an NT cutter, and after extruding the sample by 7 mm with anErichsen tester, the film was stripped off with a cellophane tape.(Evaluation: xx: 100 squares stripped off; x: 99 to 6 squares strippedoff; Δ: 1 to 5 squares stripped off: ◯: no stripping, but strippingobserved around cut scratches; ⊚: perfectly no stripping).

Paint Coating Adhesion (secondary): After carrying out the sametreatments as above up to the electrodeposition, the sample was immersedin a hot water at 50° C. for ten days, and then, the same test as abovewas conducted.

Corrosion Resistance after Paint Coating: After carrying out the sametreatments as in the above-mentioned evaluation of coating adhesion upto the electrodeposition, the sample was held for one day. Cross cutsreaching the substrate were made with an NT cutter, and a salt-spraytest specified in JIS-Z-2371 was carried out for 20 days. The film onthe sample was then stripped off with a cellophane tape, and evaluationwas made from the maximum value of stripped film width (one side) fromthe cross-cut portion. (x: over 10 mm; Δ: 3 to 10 mm; ◯: under 3 mm).

Bare Corrosion Resistance: After sealing the sample edge and back with atape, the number of days before occurrence of 5% red rust was measuredin a salt-spray test specified in JIS-Z-2371. (xx: within one day; x:within two days; Δ: 2 to 5 days; ◯: 5 to 10 days; ⊚: 10 days or over).

The results of evaluation are shown in Table 6. While satisfactorypaintability and corrosion resistance were obtained in the examples ofthe invention, one or more properties were deteriorated in thecomparative examples which are outside the scope of the invention.

TABLE 6 Results of evaluation Film Chemical composition of filmCorrosion Bare weight Mg Ni Mn Mi + Mn Mg + Ni + Mn Coating adhesionresistance corrosion No. g/m2 % % % % % Primary Secondary after coatingresistance Example of 1 0.5 6.4 1.2 0 1.2 7.6 ⊙ ⊙ ∘ ∘ invention 2 1 5.31.6 0 1.6 6.9 ⊙ ⊙ ∘ ⊙ 3 1.2 8.8 0.5 0 0.5 9.3 ⊙ ⊙ ∘ ⊙ 4 1.5 3.6 2.0 02.0 5.6 ⊙ ⊙ ∘ ⊙ 5 2.2 2.5 2.5 0 2.5 4.9 ⊙ ⊙ ∘ ⊙ 6 2.1 2.1 4.0 0 4.0 6.1⊙ ⊙ ∘ ⊙ 7 2.5 6.3 1.6 0 1.6 7.9 ⊙ ⊙ ∘ ⊙ 8 2.5 4.3 1.8 0 1.8 6.1 ⊙ ⊙ ∘ ⊙9 1.7 4.2 1.2 3.4 4.6 8.8 ⊙ ⊙ ∘ ⊙ 10 1.7 4.4 0.0 2.1 2.1 6.5 ⊙ ⊙ ∘ ⊙ 112 3.5 0.1 2.9 2.9 6.4 ⊙ ⊙ ∘ ⊙ 12 1.4 3.0 1.4 0 1.4 4.4 ⊙ ⊙ ∘ ∘ 13 0.62.9 2.7 0 2.7 5.6 ⊙ ⊙ ∘ ∘ 14 1.6 7.7 1.0 0 1.0 8.7 ⊙ ⊙ ∘ ⊙ Comparative 11.5 4.0 0.0 0 0.0 4.0 Δ x x ∘ example 2 0.2 5.4 2.0 0 2.0 7.4 ∘ ∘ Δ Δ 31.2 0.3 5.0 0 5.0 5.3 ⊙ ⊙ ∘ x 4 1.5 0.0 2.5 2 4.5 4.5 ⊙ ⊙ Δ xx 5 1.5 1.02.0 2 4.0 5.0 ⊙ ⊙ ∘ Δ Example of 15 0.4 4.5 0.8 0 0.8 5.3 ⊙ ⊙ ∘ ∘inventior 16 0.3 2.9 0.3 2.5 2.8 5.7 ⊙ ⊙ ∘ ∘

EFFECTS OF INVENTION

According to the present invention, it is possible to obtain a phosphatetreated galvanized steel sheet having excellent corrosion resistance andpaintability which were not obtained conventionally. The phosphatetreated steel sheet according to the present invention does not useharmful substances such as hexavalent chromium and can be producedeasily and advantageously from the point of production cost and issuited for use in various applications such as automobile, home electricappliances and building materials.

What is claimed is:
 1. A phosphate-treated galvanized steel sheetexcellent in corrosion resistance and paintability, comprising aphosphate film in an amount of at least 0.5 g/m², containing at least 2wt. % of Mg, and at least 0.5 wt. % in total of Ni and/or Mn, with Mgand Ni and/or Mn being in a total amount of at least 4 wt. %, formed ona zinc or zinc alloy plated steel sheet.
 2. A phosphate-treatedgalvanized steel sheet excellent in corrosion resistance andpaintability, comprising a phosphate film in an amount of at least 1g/m², containing at least 2 wt. % of Mg, and at least 0.5 wt. % in totalof Ni and/or Mn, with Mg and Ni and/or Mn being in a total amount of atleast 5 wt. %, formed on a zinc or zinc alloy plated steel sheet.